This invention relates to full-bridge drivers, for driving permanent magnet DC motors in either direction, that are capable of braking the motor when the motor direction is to be reversed, and more particularly to such a fully integratable bridge driver that additionally includes a detecting means which during braking relies upon the back-electromotive-force (b.e.m.f.) voltage of the motor for determining the time at which the motor has come to rest for signaling end-of-braking.
It is well known to employ a full bridge capable of driving a DC motor in either of the two directions. Such bridges employ four switches in H-configuration wherein the motor load constitutes the central bar in the "H". For the four bridge switches, mechanical switches or silicon controlled rectifiers have been used that are essentially precluded from being formed in an integrated circuit.
It is also well known to make full-bridge motor drivers using bipolar or field effect transistor switches. Such transistor bridge drivers have included a braking means wherein at least one of the bridge transistors is utilized to "short" the motor, i.e. place a low impedance across the motor. A means for detecting the end of braking in such prior art drivers is rarely provided but in cases where such a feature is included, it is by employment of at least one braking-current sense resistor. The sense resistor is in series with the at least one bridge transistor used for "shorting" the motor. When the motor comes to rest and the armature b.e.m.f drops to zero, the "short-circuit" motor current drops to zero as does the voltage across the sense resistor. And, when braking is required in both motor directions, two sense resistors are required.
However, the use of such series connected sense resistors has numerous disadvantages. The main motor current must also flow through the sense resistors, therefore dissipating useless power and unwanted heat. Additionally, sense resistors are generally of very low value and difficult to integrate. Thus sense resistors, used in conjunction with integrated driver circuits of the prior art, are for these reasons commonly supplied as expensive discrete resistors, external to the integrated circuit, and thus become a source of a substantial increase in the cost of the driver circuit.
It is an object of this invention to overcome the above-noted short comings of the prior art and to provide a full-bridge driver circuit for permanent magnet DC motors, which driver circuit is fully integratable and has improved means for detecting end-of-braking without the use of current-sensing resistors.